1. Field of the Invention
The present invention relates to a method of manufacturing an optical waveguide for widespread use in the fields of optical communications, optical information processing, position sensors, and other general optics technology.
2. Description of the Related Art
In general, an optical waveguide is configured in such a manner that cores serving as a passageway for light are formed in a predetermined pattern on a surface of an under cladding layer, and that an over cladding layer is formed so as to cover the cores. In particular, for the formation of the over cladding layer of a desired shape, e.g. the over cladding layer having an end portion in the form of a lens portion, a mold including a cavity having a mold surface of a shape complementary to the desired shape of the over cladding layer is used to form the over cladding layer (as disclosed, for example, in Japanese Published Patent Application No. 2008-281654).
The assignee of the present application has proposed a mold made of a light-transmissive resin which is excellent in dimensional accuracy as a mold for the formation of the over cladding layer, and has already applied for a patent (Japanese Patent Application No. 2010-126714). This mold is produced in a manner to be described below. First, a mold member of a shape identical with the shape of the over cladding layer is prepared, and is placed in a mold production container. Then, the container is filled with a light-transmissive resin, and the light-transmissive resin is cured. The cured light-transmissive resin is taken out of the container, and the mold member is removed. This provides a mold made of a light-transmissive resin in which a hollow resulting from the removal of the mold member serves as a cavity for the formation of the over cladding layer.
An optical waveguide is produced using the above-mentioned mold in a manner to be described below. First, the cavity of the mold is filled with a photosensitive resin for the formation of the over cladding layer. Then, cores formed on a surface of an under cladding layer are immersed in the photosensitive resin, and the under cladding layer is pressed against the mold. Next, the photosensitive resin is exposed to light through the mold, whereby the photosensitive resin is cured and formed into the over cladding layer. Thereafter, the mold is removed. This provides an optical waveguide including the under cladding layer, the cores, and the over cladding layer.
In the step of forming the over cladding layer in the optical waveguide, the photosensitive resin for the formation of the over cladding layer is in general charged in rather greater amounts into the cavity of the mold to prevent air from becoming trapped in the photosensitive resin when the under cladding layer with the cores provided thereon is pressed against the mold. Thus, when the under cladding layer is pressed against the mold, an excess amount of the photosensitive resin tends to spill out of the cavity, so that the spilling photosensitive resin is interposed between the mold and the under cladding layer around the cavity. The photosensitive resin interposed between the mold and the under cladding layer tends to be cured as it is when subjected to the exposure process, thereby forming fins or burrs. The fins or burrs are cut off after the mold is removed. However, the fins or burrs formed on the lens portion of the over cladding layer cannot be cut off along a lens-shaped curved surface of the over cladding layer to result in a tendency toward insufficient lens performance.
Also, the presence of the photosensitive resin between the mold and the under cladding layer as mentioned above causes the mold to be shifted a distance corresponding to the thickness of the photosensitive resin from its proper vertical position. This produces the over cladding layer with a height greater than its design value to give rise to an improper positional relationship between the cores and the over cladding layer. As a result, a light beam emitted from a distal end of each of the cores is not appropriately narrowed down by the lens portion at an end of the over cladding layer, but exits the lens portion while being widened. This decreases the intensity of received light (or light propagation characteristics) on a light-receiving side. Additionally, the fins or burrs have thick portions and thin portions, which cause variations in the characteristics of the product (the optical waveguide), impairing the uniformity of the characteristics. The formation of the over cladding layer by the use of the mold still has room for improvement in this regard.